Batteries are being used in applications that require short duration “bursts” of energy. Exemplary applications include smart credit cards, hotel key cards, and cellular telephones, such as enabling a camera light bulb in the cellular telephone to flash. Some battery types, referred to as micro-energy battery cells, are extremely thin, for example less than about 150 microns thick. These batteries have very low capacity, such as 0.5 to 2 mA-hours, but can deliver 50 to 200 mA, relatively high current for their size. Power circuits are designed to prevent the battery voltage from dropping below a minimum threshold voltage. This process can be used to prevent damage to the battery and/or to maintain a minimum voltage level so that it can be used for other applications to which the power circuit is coupled, such as system power.
A conventional solution for maintaining a minimum threshold voltage is to couple a switch and a resistor in series between the battery and a storage capacitor. The storage capacitor is charged by switching on the switch, and the resistor prevents the battery from dropping below the damage threshold. Once the capacitor is charged to a predetermined level, e.g. fully charged or some lower charged level, then the load is enabled to receive high surge current from the charged capacitor for a short duration of time, while not pulling the battery down below the minimum threshold voltage.
FIG. 1 illustrates a conceptual schematic diagram of a conventional power circuit used to provide a current surge from a battery to a load. The power regulation circuit is designed to prevent the battery voltage from dropping below a predetermined voltage level. The power regulation circuit 2 includes a battery 8, a charge circuit 4 coupled to the battery 8, a regulator circuit 6 coupled to the charge circuit 4, and a load 30 coupled to the regulator circuit 6. The charge circuit 4 includes a transistor 10, a resistor 12, and a capacitor 14. The battery 8 is a power source, conceptually represented as voltage source 16. The battery 8 also has a battery impedance, conceptually represented as resistor 18. As used herein, reference to a “battery voltage” refers to the output terminal voltage at resistor 18. The resistor 12 represents an added resistance between the battery 8 and the capacitor 14. The combined impedance of the transistor 10 and the resistor 12 prevents the battery voltage from dropping below a voltage where damage to the battery occurs or to maintain a minimum voltage level for other applications. In alternative configurations, the transistor 10 and the resistor 12 are replaced with a single transistor having an impedance large enough to have the same combined impedance of the transistor 10 and the resistor 12. The transistor 10 functions as a switch that enables current flow from the battery 8 to the capacitor 14. The regulator circuit 6 includes a transistor 22, an operational amplifier 20, resistors 24 and 26, and a capacitor 28. The load 30 is coupled to the capacitor 28. The transistor 22 functions as a switch that enables current flow from the charge circuit 4 to the load 30.
Before switching on current to the load 30, the capacitor 14 is first charged by turning on the switch 10. Once the capacitor 14 is charged, the load 30 is turned on, and the energy stored in the capacitor 14 is delivered to the load 30 and regulated while the voltage of the capacitor 14 is discharged. This ensures that the voltage of the battery 8 is not pulled down below a threshold voltage where the battery 8 may be damaged or to maintain a minimum voltage level for other applications. The regulator 6 can either be always turned on, in which case turning the load on and off enables current flow, or the regulator 6 can be turned on and off to enable current flow from the charge circuit 4 to the load 30. In general, the capacitor 14 is charged before the load 30 is turned on or before the regulator 6 is turned on. At or below the damage threshold, the battery loses capacity and eventually becomes inoperative. Maintaining the battery voltage above the damage threshold enables the battery to be used for thousands of cycles.
The power circuit 2 can be used in many applications including sensors and smart credit cards. In the case of a smart credit card, the battery is inside the credit card. The credit card also includes a microprocessor and transmitting circuitry.
A disadvantage of the power circuit 2 is that the resistance of the resistor 18 within the battery 8 is highly dependent on temperature. Within the typical operating temperature range of the power circuit, such as between minus 25 degrees Celsius to 85 degrees Celsius, the resistance can vary from a few ohms at the high end of the temperature range, such as 5-10 ohms, up to thousands of ohms at the low end of the temperature, such as 12K ohms. This is a significant range of resistance. The higher the ambient temperature, the lower the instantaneous resistance of the resistor 18. The lower the instantaneous resistance, the greater the current output from the battery. The current output from the battery needs to be limited so as not to drag down the voltage of the battery. The power circuit 2 is designed to account for the worst case current output condition, which corresponds to the lowest temperature at which the power circuit 2 is to be operated. The combined impedance of the transistor 10 and the resistor 12 slows the current flow from the battery 8 to the capacitor 14 to prevent the battery voltage from dropping to the damaging threshold voltage. To account for the broad variation in battery resistance due to temperature, the resistor 12 is designed in accordance with the worst case temperature condition, which corresponds to the low temperature in the expected operational temperature range. The lower the temperature, the higher the resistance of the resistor 18, and therefore the greater the need for a larger resistor 12 to account for the increased output impedance of the battery 8. As such, the application for which the power circuit 2 is used dictates the size of the resistor 12. For example, if the power circuit 2 is to be used in an application where the low end of the operational temperature range is going to be as low as ten degrees Centigrade, a first resistor is used. If the power circuit 2 is to be used in another application where the temperature is going to be as low as zero degrees Centigrade, then a second resistor is used, where the second resistor has a greater resistance than the first resistance due to the lower expected temperature. So the power circuit 2 includes the resistor 12 with a resistance designed for the worst case condition, which is the lowest expected temperature at which the power circuit is to be used.
The larger the resistor 12, the longer the time to charge the capacitor 14. However, when the power circuit 2 is used during non-worst case scenarios, at for example 20 degrees Centigrade, the length of time to charge the capacitor 14 is still dependent on the resistor 12 designed for the lower worst case temperature. Charging the capacitor 12 while at 20 degrees Centigrade takes almost as long as charging the capacitor 12 while at the worst case temperature, for example zero degrees Centigrade. The charge time for this case is longer than if the power circuit were designed for a worst case temperature of 20 degrees Centigrade. It would be advantageous to reduce the charge time for non-worst case conditions.